a) Field of the Invention
The present invention relates to an oxidation process for semiconductor wafers, and more particularly to a thermal oxidation process for semiconductor wafers.
b) Description of the Related Art
A vertical type thermal treatment furnace such as shown in FIGS. 6 and 7 is known in which semiconductor wafers are subjected to a wet oxidation process.
A wet oxidation process is an oxidation process to be performed in an atmosphere containing water vapor, commonly using a mixed gas of water vapor and oxygen. In contrast thereto, a dry oxidation process is an oxidation process to be performed in a gaseous atmosphere containing only oxygen or oxygen and an inert gas. In place of a method of generating a gas containing water vapor by bubbling the gas in pure water, a method of generating a gas containing water vapor by burning hydrogen in an oxygen gas has been used recently.
In FIGS. 6 and 7, reference numeral 10 represents a process tube disposed upright which is made of, for example, a circular quartz tube. Although the top of the process tube 10 is closed, the top is shown broken in FIG. 6 so as to expose the interior of the process tube 10.
Two gas supply pipes 12a and 12b made of quartz and branched from a lower gas supply pipe 12 extend upward along the side wall of the process tube 10. The gas supply pipe 12 is coupled to an external gas burner 14.
The external gas burner 14 is supplied with hydrogen H.sub.2 and oxygen O.sub.2, and burns the hydrogen to generate oxidizing gas containing water vapor. The oxidizing gas is introduced into the upper space of the process tube 10 via the gas supply pipes 12a and 12b.
A gas exhaust pipe 16 is provided at the lower portion of the process tube 10. The gas introduced into the process tube 10 is exhausted from the exhaust pipe 10 to an exhaust or evacuation system as an exhaust gas G. As shown in FIG. 7, a heater 18 is mounted surrounding the gas supply pipes 12a and 12b and process tube 10.
In operation, semiconductor wafers WF supported by a wafer holder (not shown) are trailsported into the process tube 10 from the bottom thereof. The bottom of the tube 10 is closed by a lid formed integrally with the wafer holder. A wet oxidation process is featured by a fast oxidation speed, a small defect density, and other good oxide properties. For example, in the case of a MOS IC, this process is used for forming a field oxide film, a gate oxide film, or the like.
With the vertical type heat treatment furnace described above, O.sub.2, is introduced into the external gas burner 14 and thereafter, H.sub.2 +O.sub.2 are introduced. H.sub.2 is then burnt so that an oxidizing gas containing O.sub.2, and water vapor, is supplied to the furnace. In this case, it takes about one minute to stabilize an oxygen flame after O.sub.2 was introduced into the furnace. During this period, partial pressures of the gas contents (oxygen, water vapor, hydrogen) are unstable. Therefore, wafers are exposed in an unstable atmosphere while forming an initial oxide film where an oxidation speed is relatively fast, resulting in an increased variation of oxide film thickness and a lowered reproducibility. It is difficult to form an oxide film, particularly a very thin oxide film, at high yield.